1. Field of the Invention
The present invention relates to a technology for assuring data read by an offset-reading retry process by offsetting in an inner direction toward a center of a storage medium and an outer direction toward a circumference of the storage medium, when a read-error occurs near a track center of the storage medium.
2. Description of the Related Art
In recent years, to increase the speed and reliability of reading data, a read-head of a magnetic disk device has become smaller, and even smaller than a write-head. This is because a core of the read-head can be smaller without decreasing the reading capability by using Magneto Resistive (MR), Giant Magneto Resistive (GMR), or Turning Magneto Resistive (TMR), whereas the core of the write-head can be made small to a certain extent because too small size of the write-head degrades the signal to noise ratio (SNR) of the write-head.
Ideally, the data is written along a track centerline of the magnetic disk. However, an arm that supports the heads vibrates slightly and accordingly the read-head also vibrates, resulting in an offset-writing in which the data is written away from the track centerline.
When new data is written on top of old data by the offset-writing, both the old data and the new data can be readably present near the track centerline. This is due to an offset of the offset-writing. In other words, this is caused by the new data being offset-written over a part of the old data readably left because the old data is not completely overwritten with the new data due to the deviation of the written data from the track centerline.
When the new data is offset written on top of the old data, an edge of the offset-written data can be positioned near the track centerline. When the read-head tries to read-data without offsetting, a read-error can occur.
As a method for addressing such a read-error, a method of reacquiring data by changing an offset amount according to read gain is disclosed in Japanese Patent Application Laid-open No. 2004-134077, for example.
Furthermore, Japanese Patent Application Laid-open No. 2001-118343 discloses a magnetic disk that can read-data without performing offset-reading if a read-error occurs when the data can be recovered by offset-reading, once the data in a sector that can be read by offset-reading and the neighboring sectors are saved and rewritten to the original sector without offsetting.
However, with the conventional techniques represented by the disclosures in Japanese Patent Application Laid-Open No. 2004-134077 and No. 2001-118343, when new data is overwritten on old data by offset-writing, and both of the new data and the old data can be read due to the difference in the offset amount, it is not known which the new data is.
Specifically, when new data is offset-written in an outer direction toward the circumference of the magnetic disk over the old data written on the track centerline with an offset amount of zero, data read by offset-reading in an inner direction toward the center of the magnetic disk is the old data, and data read by offset-reading in the outer direction is the new data.
Under these conditions, it cannot be determined which data is the correct data at the stage where data is read for the first time. Thus, it is required to assure the data read by offset-reading when a data read-error occurs on the track centerline.